The invention relates to a hot-tinned wire for electrotechnical purposes, made of copper or copper alloys with a dual-layer coating of tin or tin alloys.
The purpose of hot-tinning copper hook-up wires is to provide the wires with a firmly adhering coating of tin or tin alloys so as to assure a perfect and reliable solder connection when automatic soldering operations with soldering times of about 1 second are carried out. Freshly hot-tinned copper hook-up wires generally meet this requirement. However, their solderability decreases greatly after a long storage period, in consequence of which the soldering times increase up to more than 20 seconds. This deterioration of solderability is attributable to the formation of a Cu.sub.3 Sn phase which is not, or only poorly, wettable by the solder. This phase forms at the boundary between copper and tin, its crystallites growing through thin tin coatings. Particularly in wires having eccentrically applied coatings, the Cu.sub.3 Sn phase will very quickly penetrate to the surface in the area of the thinnest coating thickness.
For this reason, various attempts have previously been made to retard the penetration of the Cu.sub.3 Sn phase to the surface of a hot-tinned wire. The first efforts were directed towards producing coatings as uniformly thick as possible, with a minimum coating thickness between 3 and 10 .mu.m. For example, it is known from German Offenlegungsschrift No. 1,957,032 to profile the molten tin coating of a wire by means of a stripper nozzle, the aperture profile of which is bounded by a train of waves. The subsequent smoothing and uniform distribution of the tin then comes about automatically due to surface tension. Further, it is known from German Pat. No. 1,621,338 to improve the wettability of a wire by applying a thin first coating of tin or tin alloys and then subsequently applying a uniform thicker second coating. However, thick hot-tinning with concentric and uniformly applied coatings does not retard the formation and the penetration of the Cu.sub.3 Sn phase. It is only the break-through of the Cu.sub.3 Sn phase to the surface which is being retarded due to the greater expenditure of tin, i.e. greater distances at the same diffusion speed. Moreover, the realization of good concentricity of the coatings in hot-tinning operations employing wire speeds about 1.5 m/s will only be imperfect.
Therefore, it is an object of the invention to provide a hot-tinned wire of copper or copper alloy in which the formation and penetration of a Cu.sub.3 Sn phase is effectively retarded and in which good solderability according to the solder ball test (DIN 40046 Sheet 18) with enveloping times of less than 2 seconds after a heat and time stress between 4 and 96 hours at 155.degree. C in air is assured.